Practical Considerations in Choosing and Using Telescopes for CCD Imaging

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Introduction

This talk discusses the characteristics of different kinds of telescopes when applied to CCD imaging. It includes a discussion of the pros and cons of different designs in imaging applications.

This is an introductory level discussion. It doesn’t delve into optical theory, but focus on practical considerations of using the kinds of scopes available to amateurs.

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Five Factors to Consider in evaluating telescope used for CCD Imaging

1. Focal Length2. Focal ratio3. Aberrations4. Optical Quality and Design5. Size of Central Obstruction

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Focal Length

Determines Image scale and field of view.

Degree to which scope is affected by seeing.

Guiding accuracy

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CCD Pixel Size

Arc sec/pixel = Pixel size (microns)/focal length(mm) * 206

This formula provides good approximations in the focal length ranges typical of amateur telescopes. It is not accurate at very short focal lengths (e.g. 50mm).

Resolution is maximized, more or less, when the smallest angular detail covers two pixels.

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Popular Amateur Telescope Types1. Refractors

AchromatsApochromats

2. Newtonian Reflectors.Modified Newtonians

3. Schmidt-Cassegrains4. Maksutov Newtonians5. Maksutov Cassegrains6. Ritchey-Chretien7. Other Cassegrains

Classical CassegrainsDall-Kirkham

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Achromatic Refractors

Run the gamut from cheap dept. store models to well-made, high quality offerings.Because of their small apertures and closed tubes, they cool down to ambient temperature relatively quickly. Problems with tube currents are minimal.The closed tube also means maintenance issues are minimal. Relatively inexpensive.

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Achromatic Refractors (continued)The main problems with using achromats for CCD imaging is thatthey suffer from chromatic aberration.

This occurs when different wavelengths of light reach focus at different points. As a result, only some of the wavelengths of light are in focus. Others are out of focus.

Chromatic aberration is minimal in good quality achromats that have focal ratios of f/15 or larger. Even at f/10 chromatic aberration is often not too objectionable in high quality models. However, these aberrations are more evident in CCD images than in visual astronomy.

Brighter stars in images taken through achromats often have stars that appear fuzzy or show false color.

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Omega Centuri Orion ST80 f/5 Achromat–Jim Edlin Image

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Achromatic Refractors (continued)You can overcome chromatic aberrations and still produce greatCCD images with fast achromats through the use of filters.

Filters allow a narrower band pass of light to pass to the CCD chip. As a result, problems associated with bringing different wavelengths of light at different points are lessened. Yellow and green filters can help, especially in monochrome images. Red filters can produce pleasing results on the right objects.

Really narrow band pass filters such as H Alpha filters can produce results similar to those delivered by APO refractors when used with the same filter.

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Orion ST-80 80mm f/5 achromat. M8 with H alpha filter. Jim Edlin image.

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Apochromatic Refractors

Apochromats bring all wavelengths of lights to focus at more or less the same point, by using special glass elements, one or more of which may contain fluorite.The chromatic aberration which plague achromats is largely eliminated. There are few competing designs that can match or exceed their ability to produce such crisp, wide fields of view, at any price. For this reason, my personal bias is to try to get the fastest f ratio APO I can get at a given aperture. This gives the largest FOV.

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Apochromatic Refractors (continued)

Apochromats are among the easiest of telescopes to use for CCD imaging.

1. They are easy to focus.2. Seldom affected by tube currents.3. Aren’t limited by seeing to the same extent as other

longer focal length designs.4. Collimation is seldom a problem.5. The relatively short focal lengths of these scopes

makes them relatively easy to guide.These qualities would make them an ideal scope for CCD imaging beginners except they are very expensive.

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Apochromatic Refractors (continued)

There are a number of excellent offerings out there including, but not limited to those from Astro-physics, Televue, Takshashi, TMB, and TEC.

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Apochromatic Refractors (continued)

Limitations of ApochromatsThey are priceySome may have insufficient “in focus” to accommodate all the accessories that you may want to place in the optical train.Chromatic aberration is never completely eliminated. Some of itis typically present at the fringes of images, especially on those taken through some of the newer wide field CCD chips. Personally, I don’t find the limited chromatic aberration to be objectionable.6-7” aperture is a practical upper limit for most people. Larger aperture scopes are expensive and have such long tubes that most people prefer competing telescope designs.

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M31 with Tak FSQ 106 4” f/5 APO

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Schmidt-Cassegrains

These are probably the most popular type of amateur telescope.More people begin imaging with SCT’s than with any other telescope design.They provide relatively large apertures, in a very compact tube, at relatively affordable prices.These generally provide slow focal ratios (f/10 or more) and longer focal lengths (2000mm +) unless used with focal reducers.

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Advantages of Schmidt-Cassegrains

Because they are widely used, myriad accessories have been developed for SCT’s including those that aid in imaging. This include zero-image shift focusers, flip mirror systems, focal reducers, and numerous accessories for coupling CCD cameras to the scope.SCT’s have more “in-focus” than most other competing designs. This means that you can add a number of accessories into the optical path of an SCT. For example, you can add adaptive optics accessories such as SBIG’s A0-7 along with a filter wheel and focal reducer and still bring an image to focus. This isn’t the case with most competing designs.Focal reducers are readily available for SCT’s that allow you to reduce the focal length by factors of .63, ½, or 1/3 depending on the reducer. These enhance the versatility of SCT’s by allowing change the image scale and field of view.

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Disadvantages of SCT’sSCT’s suffers from significant coma and field curvature.SCT’s use a moving primary for focusing. Because of the mechanical design, the mirror tends to shift or tilt slightly during focusing. It also sometimes shifts when you point to different positions in the sky.SCT’s are difficult to focus. It is difficult to tell when you are in focus. A separate electronic focuser for close focusing, that does not move the primary, such as the JMI NGF-S or Optec TCF is highly desirable in this context. So is software such as Focus max which can automatically focus an SCT in conjunction with one of these focusers. SCT’s are highly sensitive to collimation and lose collimation easily. Mirror shift can contribute to this problem. SCT’s aren’t very easy to collimate. “Bob’s knobs” can help make collimation easier.The combination of features described above make the SCT one of the more difficult telescope designs to use in CCD imaging.Quality control in SCT manufacture has been a problem in the past with the result that some of these scopes are significantly better than others. In recent years, quality control appears to have improved.

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M27 taken with a C-11 at f/6.3

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Newtonian ReflectorsClassical Newtonian reflectors are well suited for CCD imaging.

1. Newtonian secondary obstructions tend to be smaller than those of SCT’s resulting in higher contrast.

2. They tend to hold collimation quite well.3. Focusing is fairly easy.4. Many are available with fast f ratios in the f/4 – f/6

range.5. The primary off axis aberration of Newtonian’s is

coma. This problem increases inversely with focal ratio.

6. Diffraction spikes

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Newtonian Reflectors (continued)In order to correct for coma in Newtonian reflectors, a variety of correctors have been developed over the years.An example of this is a two-element corrector offered in Takahashi’s MT line of Newtonian’s which significantly reduces off-axis aberrations. Televue’s Paracorr is another example.Takahashi Epsilon line of astrographs is worthy of note as instruments especially made for imaging. These utilize special hyperbolic primary mirrors and a four element field corrector flattener that results in ultra fast f/3 Newtonian systems. These are capable of delivering pinpoint star images across a 35 mm film sized field. This is perhaps the closest thing to a Schmidt Camera available for amateur CCD imaging. It produces stunning images that compare favorably with the best offerings from APO refractors.

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Brian Lula NGC 6992 Ha/HaLRGB image with 20” f/5 Homebuilt Newtonian

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Maksutov NewtoniansThese employ a sharply curved meniscus corrector that lies inside the field of curvature of the primary. These designs yield reduced off-axis aberrations compared with a classical Newtonian. They typically have very small secondary obstructions which enhances contrast. These are good planetary scopes. F ratios are typically in the f/5 – f/8 range.While aberrations aren’t eliminated entirely, Mak-Newts produce sharp images across a wide field that rival APO refractors of similar aperture and at a much lower cost.

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Mak-Newt 6” f/6: NGC 7293 – Helix Nebula

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Maksutov-Newtonians (continued)

The thick corrector plate and the primary mirror can take cause the scope to take a long time to reach thermal equilibrium with the outside air. Cooling fans can help.Caveat: Make sure the scope you want has enough “in-focus” before buying. Some Mak-Newts have a problem in this area.

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Maksutov-Cassegrain

Employs a sharply curved meniscus lens to offset spherical aberration in the primary.Light is brought to a Cassegrain focus.Very Compact designRelatively easy to buildMany high quality scopes out there including offerings from Intes, Intes-Micro, AP, and TEC.

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Maksutov-Cassegrain continuedThese tend to have long focal ratios (f/10 – f/15) except for special versions made for imaging.Secondary obstruction tends to be a little less than Schmidt-Cassegrains, but larger than those found in Mak-Newts.There are few shorter focal ratio Mak-Cass’s on the market because those scopes have large secondary obstructions that limit their effectiveness in visual astronomy, especially on the planets.One exception is the Intes MK69 which is a relatively inexpensive 6: f/6 Mak-Cass. This has a secondary obstruction of more than 50% which is nonetheless an excellent deep sky imager.

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Bob Holzer’s M81 with 6”TEC f/12 Mak-Cass imaging at f/8.14

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Ritchey-Chretiens

This is the “Cadillac” of scopes when it comes to imaging at longer (1700mm and greater) focal lengths.This design virtually eliminates coma which makes them ideal for imaging applications. You get nice round stars to the edge of the fieldThe optical tube is more compact than Newtonians of like aperture. F ratios are moderately fast compared with SCT’s.Open tube minimizes problems with dew.

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Ritchey-Chretiens - DisadvantagesField curvature is an issue as stars in the center and sides come to focus at different points.The mirrors in a Ritchey-Chretien are highly aspheric which makes them expensive to build.Secondary obstruction tends to be quite large. Around 40%. You might not choose this scope if your primary interest is planetary imaging.There is a certain optimum distance that must be maintained between the secondary and primary mirrors. If this spacing is allowed to deviate much more than a few millimeters, significant deterioration of performance will result due to spherical aberration. This has implications when you enter new items in the optical train such as focal reducers.

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Brian Lula M106 image with 20” f/8 RCOS

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Classical CassegrainsThese provide long focal lengths in a compact tube.They exhibit coma similar to Newtonian’s of equal focal length but have much more field curvature.The offerings in this design are somewhat limited.The nature of the Cassegrain design is such that it’s fairly easy to build such a scope such that can be used interchangeably as a Newtonian. This is accomplished by swapping out the convex secondary, replacing it with an elliptical diagonal mirror, and installing a Newtonian focuser. Takahashi’ CN-212 offers is an example of this scope. It offers 212 mm aperture that is f/12 at the Cassegrain focus and f/3.9 at the Newtonian focus.

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Dall-Kirkham CassegrainsThese are easy to manufacturer because of their elliptical primary’s and spherical secondaries. This makes them relatively inexpensive compared with other designsThey suffer much more from coma than Classical Cassegrains.Takahashi’s Mewlon line consists of high quality Dall-Kirkham’s where aberrations are minimized.

1. Long focal length (f/12 or so) and compact tube.2. Aperture mask at the edge of the tube coupled with a slightly

oversized primary.3. They are well built and . 4. Very sensitive to collimation errors, but easy to collimate & hold

collimation quite well.5. Smaller Mewlon’s focus with a moving primary and suffer from

mirror flop, but to a lesser extent than SCT’s. Larger Mewlons focus by moving the secondary mirror and have no mirror flop.

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Lunar Southwestern Quadrant taken with a Takahashi Mewlon 180mm f/12

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Parting thoughts

There is no best telescope design for CCD imaging. If you can reach focus, almost any scope can deliver great CCD images if you work around its particular limitations.In general, the mount is more important than the optics. Get a good quality tracking mount whose carrying capacity is well in excess of weight you intend to place on it. Overkill is good.